1. Field of Invention
This invention relates to portland cement manufacture, specifically to an improved process of thermal reduction and decomposition of gypsum when used as a source of lime in portland cement.
2. Discussion of Prior Art
In the massive masonry constructions of the Egyptians we meet with our present day system of uniting blocks and slabs with a mortar consisting of a mixture of sand with a cementations material. While the typical Egyptian mortar has been generally described by writers on Egypt as burnt lime, even when found in buildings as old as the great pyramids chemical examination shows that the Egyptians never used lime until the Roman period and that the cementing material was always obtained by burning gypsum.
Scientists have studied the decomposition of calcium sulfate since the turn of this century. Lunge in 1903 was the first to suggest that calcium sulfate could be directly decomposed to sulfur dioxide by heating with clay in a shaft kiln, with cement clinker formed as a by-product.
In 1908, Hofman and Mostowithsch published studies showing the effect of temperature on speed of decomposition of calcium sulfate with additives. Actual development and process implementation took place in Germany. U.S. Pat. No. 1069191, 8/1913 to Von Schlippenback already illustrates a calcium sulfate burning process. In 1915 W. S. Mueller investigated the decomposition of calcium sulfate with additives in the laboratory, but the successful development of the process to plant scale was carried out by H. H. Kuhne. Thus, in 1918 a sufficiently concentrated sulphur dioxide gas and good cement clinker were produced The Muller-Kuhne technology of cement-sulphuric acid manufacture was relatively widely adopted in Europe starting in the 1920's with most successful operations by Marchon in England at Whitehaven and Billingham, England. Other countries which have built plants based on this technology are France, Austria, South Africa and Poland. The situation in England is typical of all cases where this process has been used and can be summarized as follows:
a) sulphuric acid was needed to support other chemical operations.
b) The British government limited sulphur imports due to a balance of payment problem and refused to allow additional sulphur imports.
c) Large reserves of gypsum, and coal were locally available at attractive prices.
d) "Home"-produced chemicals would be protected by duties on imported chemicals.
To date this process has not been able to "stand alone" when the alternative was to burn sulphur to produce sulphuric acid without help from economic forces outside of those normally used to evaluate the merits of a chemical process. This situation is unchanged today for the gypsum based sulphuric acid-cement plants operating in both South Africa and Poland.
Essentially, the process comprises heating the gypsum or anhydrite with reductant to produce sulphur dioxide and lime. Other additives such as silica, ferric oxide, and alumina are added as required for portland cement manufacture.
Three reaction stages are recognized. In the reduction stage carbon reacts with one fourth of the calcium sulfate source at 900.degree.-1000.degree. C. EQU 4CaSO.sub.4 +2C=CaS+3CaSO.sub.4 +2CO.sub.2 ( 1)
This is followed by a decomposition step, where calcium sulfide reacts with the remaining calcium sulfate at a considerably higher temperature (1200.degree. C.) to form sulphur dioxide: EQU CaS+3CaSO.sub.4 =4CaO+4SO.sub.2 ( 2)
The overall reduction - decomposition course corresponds to the reaction EQU 2CaSO.sub.4 +C=2CaO+CO.sub.2 +2SO.sub.2 ( 3)
The standard recombination reactions to form the clinkering compounds i.e., dicalcium and tricalcium silicate begin when the lime is completely desulfurized and the temperature increased to 1400.degree. C. to 1450.degree. C.
All of these reactions take place in a long kiln. The most significant improvements have addressed the treatment of the raw materials and improved pre-heating and heat exchanging equipment at the feed end of the kiln, for example, German patent 1206193 for 1970; German patent 1285864 for 1971, and U.S. Pat. No. 3,865,602, Feb. 11, 1975 to O. S. W.
Calcium sulfate decomposes at 1100.degree.-1200.degree. C. while decarbonation of CaCO.sub.3 occurs at 900.degree. C. The endothermic heat of reaction of the sulfates decomposition is 36% higher than in the case of its carbonate counterpart.
The higher heat of reaction and temperature are not a major problem in themselves but their combined effect being considerably magnified by the poor heat transfer characteristics of the rotary kiln is a major disadvantage.
In effect, the reduction reaction, the decomposition reaction and the clinkering reactions all take place in a rotary kiln. In the rotary kiln the material enters the upper or more elevated end, and due to the rotation of the kiln, passes downwardly. All of the fuel is injected and is burned at near the lower end, typically called the "burning zone". The formation of clinker also takes place at the lower end where it benefits from heat of radiation provided by the flame. In this manner two important requirements are met: the mass of solids reaches about 1450.degree. C. at which temperature incipient melting occurs while all particles are equally exposed to the heat of radiation of the flame due to the tumbling action and mixing provided by the rotating kiln. Under this conditions the recombination solid reactions between calcium and silica tetrahedra known as clinkering take place in a most uniform manner and a homogeneous portland cement is produced.
The combustion gases passing upwardly from the clinkering zone then are expected to provide the heat for the decomposition reaction which is strongly endothermic (64K cal per mole) which creates a "heat sink" within the solids mass i.e., a cooling effect caused by the endothermic reaction. Additionally heat needs to be provided so that the solids reach 1200.degree. C. which is the decomposition reaction temperature.
Unfortunately, heat transfer between the combustion gases and the mass of solids is relatively poor. Also, we have to keep in mind that while heat of radiation is being transmitted at the rate of the fourth power of the temperature differential between the flame and the wall of the kiln at the clinkering or lower end of the kiln, the mass of solids undergoing the decomposition reaction can only count on convection and conduction which is of the order of the first power of the temperature differential between solids and gases.
Characteristic temperature profiles for both gases and solids are shown on FIG. 3--Curves 3A and 3B. It is noticeable that the curves largely deviate from each other in the portion of the kiln undergoing the decomposition reaction. In fact for a substantial portion of the kiln length the temperature of the solids is almost constant and the decomposition and desulfurization of calcium sulfate almost resembles an isothermal process such as the boiling of water. Furthermore, another important disadvantage of the prior state of the art is posed by the proximity of solids blanketed by said endothermic decomposition reaction to the clinkering or burning zone of the kiln which creates a cyclical operating and control problem i.e., having the burning zone of the kiln intermittently "cooled" by the endothermic decomposition reaction. The word "cooled" is used to indicate the effect on the reacting solids of high amounts of heat being absorbed at the wrong kiln location or zone.
Continuing upwardly in the kiln towards the feed end, the combustion gases enter the area of the kiln where the reduction of calcium sulfate to calcium sulfide takes place. In order to accomplish this reaction at a minimum temperature and retention time careful control of reduction variables is necessary. This is not possible in the kiln process since the optimum reduction atmospheres cannot be controlled. In fact oxidizing conditions are more suitable in the other two reaction zones already discussed. In order to maintain reduction atmospheres at the feed end, efficiency is sacrificed in the thermochemistry of the rest of the kiln.
Under reducing conditions we may have the following set of reactions: ##EQU1## Now under oxidizing atmospheres we can have: ##EQU2## This reaction as well as the combustion of carbon monoxide can occur at 750.degree. C. to 800.degree. C. thus using up carbon over and above the ratio of 0.5 moles of C per mole of CaSO.sub.4 stipulated by Kuhne.
Also the same gas-solid heat transfer limitations pointed out for the other zones are even more critical at the feed of the kiln where we are trying to carry the more complex reduction reactions. Thus, the reduction to CaS is limited by its inclusion at the feed end of the kiln which is a serious disadvantage. Additionally the kinetics of various reduction reactions are different and we should be able to control and optimize conditions which aim at the reaction which offers the most favorable kinetics.
It should be obvious to those familiar with the state of the art that the only way to overcome these problems in the Muller-Kuhne type technology is by adding substantial length to the kiln and decreasing its through put as a higher retention time is in order. It is also required to maintain a very high flame temperature and burning zone temperature, as well as the need of an excessive amount of fuel. All of the above requirements translate in serious operating and maintenance cost disadvantages.
Some of the prior art as mentioned above attempt to overcome the waste of heat energy with the gases exiting the kiln which is an advantage. However, they do not deal with the sources of the problem, most of which, as above explained are related to attempting to carry all three reactions in the rotary kiln.
In view of the above it may not be so striking that the clinker yield of similar kiln units is typically only half as large with the calcium sulfate process as when using calcium carbonate in a normal portland cement raw mix.
In addition to those prior art references which follow the orthodox Muller-Kuhne technology there are other desulfurization techniques worthy of mention. In U.S. Pat. Nos. 3,087,790 for 4/1963, 3,260,035 for 7/1966 and U.S. Pat No. 4,102,989 for July 25, 1978 to Wheelock et al., the thermal reduction and decomposition of gypsum are also limited by their taking place in a single furnace as we already explained for the Kuhne technology although the more efficient fluidized bed reactor is used. U.S. Pat. No. 4,312,842 for Jan. 26, 1982 to Wilson et al., also teaches the combination of the reduction and decomposition (desulfurizing) reaction in a fluidized bed reactor. In this art reduction is carried at a high temperature and low reaction rate to accommodate the decomposition reaction which leads into high capital cost of equipment. In this case the objective is reaction of EQU 2CaSO.sub.4 +C=2CaO+CO.sub.2 +2SO.sub.2
which takes 1.5 hrs of retention time at 1200.degree. C. while reduction reaction ##EQU3## already mentioned above in our discussion of the Kuhne technology takes place in 15 to 20 minutes when reduction conditions are geared to just this reaction. Also, the reaction to CaS uses one fourth of the gypsum while CaS later reacts with the remainder of the gypsum charge (3/4) in the following decomposition step. Thus, the specialized reduction step can be performed in substantially smaller equipment when the two reactions are kept separate. Portland cement clinker is not continuously produced but lime is their objective. Presumably portland cement could be manufactured from this lime, but at a great expense of reheating it from atmospheric conditions to 1350.degree.-1450.degree. C.
U.S. Pat. No. 4,503,018 for Mar. 5, 1985 to McKee teaches desulfurization of phosphogypsum in a traveling-grate type sinter machine. Although they recover sulfur dioxide for sulfuric acid manufacture their byproduct is a sinter unsuitable for the manufacture of portland cement. The main advantage claimed by this system is increased production rate of sulfur dioxide.
U.S. Pat. No. 4,608,238 for August 1986 to Wilson & Spigolon describe a mechanism for desulfurization of gypsum which combines a sintering type machine with an electric furnace. Even if the byproduct clinker obtained in this fashion were of sufficient quality for portland cement manufacture it would have been obtained by melting the raw mix at a temperature of approximately 400.degree. C. higher than that needed in the Kuhne rotary kiln process where only incipient melting is required to equalize temperature conditions throughout each nodular mass or clinker.
Another disadvantage of the above mentioned patents which use a traveling-grate is the same thermochemical limitation noted for the fluidized bed reactor process i.e., that both reduction and decomposition occur simultaneously, thus the optimum conditions for each are not provided. In the McKee patent reduction and calcination take place in the "firing" section of the grate. The Wilson patent distinguishes a reduction zone and a calcination zone, but since SO.sub.2 is produced in the reducing zone it is quite evident that both zones have the same thermochemical purpose. If the purpose of reduction were to reduce to CaS exclusively, CO.sub.2 would only be produced. EQU CaSO.sub.4 +2C=CaS+2CO.sub.2
The traveling-grate processes are likewise limited by the fact that the full amount of gypsum for complete conversion to CaO is charged as pellets at the preheating or dehydration zone and then move on to the reduction and/or oxidizing zones of the furnace, while as proposed herein only one fourth of that amount of gypsum is required by a separate reduction reactor.
This fact contributes to what has already been stated, that only simultaneous reduction and decomposition can be accomplished by the traveling-grate, sinter system.
Finally, it is a historical fact that portland cement raw mixes are better burned in a fine particle state so that homogenization to achieve the right chemical composition is not hampered. In U.S. Pat. Nos. 4,312,842, and 4,608,238 this deficiency is solved by melting in an electric furnace. In addition of the thermal disadvantage already pointed out there is a problem with the quality of the cement itself.
First of all, the clinker produced in the electric furnace is harder than regular clinker and requires a higher level of grinding energy. Then, there is the consideration of the clinker structure itself. Formation of C.sub.3 S is associated with the amount of liquid formed in a kiln at the burning zone. The fact that a melt is produced in the case of the electric furnace would indicate that a potential for the formation of a considerable amount of C.sub.3 S is inherent to this system. However, both C.sub.3 S and C.sub.2 S are unstable structures. Some of the C.sub.3 S, where high values of this compound are attained, reverts to CaO and C.sub.2 S. Under certain conditions of cooling there is a tendency to dust and conversion of C.sub.2 S from the beta form to the gamma form. The reason why gamma C.sub.2 S is formed in the case of the electric furnace melt is attributed in part to the higher crystallization temperature and molten conditions while there is a longer cooling period inherent to cement made at high temperature.
Thus melting conditions produce a more perfect crystal in the case of C.sub.2 S of the gamma form. The problem is that crystals with a deformed or strained lattice typically produced under conditions of incipient melting in a kiln burning zone are reactive, while the perfect crystals obtained in the high temperature melt are unreactive. Thus betaC.sub.2 S (made in a kiln) has an irregular structure and hydrates. GammaC.sub.2 S produced in an electric furnace melt is non-hydraulic. The irregularity of the coordination of Ca in the unstable structures, such as C.sub.3 S and betaC.sub.2 S is concomitant with the existence of structural holes. This holes are essential to the hydration processes taking place in portland cement. The structures of calcium silicates or aluminates which exhibit holes and have good hydrating properties are C.sub.3 S, betaC.sub.2 S, C.sub.3 A, C.sub.12 A.sub.7, CA and C.sub.4 AF.
The structures of calcium silicates or aluminates which do not hydrate (perfect crystals obtained from high temperature melts) and which are close in composition to the above are regular and there are "no holes". These crystals are gammaC.sub.2 S, betaCS and C.sub.2 AS. Considering all the disadvantages of using an electric furnace we intend to keep a short length of the kiln to perform the clinkering reactions (burning zone already described) in our proposed method.
In view of the above, it is easy to understand why electric furnaces have not found wide application in portland cement making. Only in specialized cases to produce high alumina cements which require very high temperatures. There is some merit, however, to the removal of P, F and other impurities from phosphogypsum in the electric furnace art. Nevertheless, impurity removal can be accomplished ahead of the reduction step more effectively, without jeopardizing the quality of the portland cement produced. Ref. Case, Dreschel patents. Also Australian, English and Indian practice.